1. Field
The present disclosure relates to optical elements, and more particularly, to optical image modulators having a PIN diode structure and methods of manufacturing the optical image modulators.
2. Description of the Related Art
In order to use a gallium arsenide (GaAs)-based optical image modulator in a three-dimensional (3D) camera, optical modulation should be able to occur at high speed over an aperture having an area of more than, for example, 4 mm×3 mm, at a frequency of several tens of MHz. An optical image modulator includes a resistance component and a capacitance component (a parasitic capacitance component). Accordingly, in order to perform optical modulation at high speed in the optical image modulator, a time constant (RC) should be small.
A resistance component included in an optical image modulator having a PIN diode includes a contact resistance between a metal that is an electrode material and a semiconductor material, a sheet resistance of each electrode contact layer, and a vertical resistance of a distributed Bragg reflection (DBR) layer. From among the resistances, the sheet resistance of the electrode contact layer is the highest. As an attempt to reduce sheet resistances of a P electrode and an N electrode in an optical image modulator having a PIN diode, there has been suggested an optical image modulator in which a fishbone-typed electrode is formed on an electrode contact layer, and a fishbone pitch is kept small in consideration of a ratio of the sheet resistance of the P electrode contact layer to the sheet resistance of the N electrode contact layer, and in consideration that a sheet resistance of an N electrode contact layer and a line resistance of a metal line of the P electrode are increased farther away from a bonding pad.
However, in such an optical image modulator, if a sheet resistance of the N electrode is about ⅕ of a sheet resistance of the P electrode, the fishbone pitch is further reduced and thus a metal shadow is increased due to a portion with the further reduced fishbone pitch, thereby reducing a light reflectance.
Meanwhile, a sheet resistance of the N electrode contact layer may be further reduced by doping silicon (Si) on the N electrode contact layer to reduce a sheet resistance of the N electrode contact layer to about ⅕ of that of the P electrode and increasing a silicon doping density. However, when the silicon doping density on the N electrode contact layer is increased to a predetermined level, the quality of the N electrode contact layer may be greatly reduced, the sheet resistance of the N electrode contact layer may be rather increased, and a breakdown voltage may be reduced. Hence, when the fishbone pitch in the P electrode is constant, a sheet resistance of the N electrode contact layer is increased farther from the bonding pad. Accordingly, a time constant of the optical image modulator may be increased, thereby making it difficult to drive the optical image modulator at high speed.